One type of thermoplastic elastomer (TPE) is a thermoplastic vulcanizate (TPV). TPV compositions are conventionally produced by dynamic vulcanization. Dynamic vulcanization is a process whereby a rubber component is crosslinked or vulcanized within a blend of at least one non-vulcanizing polymer component while undergoing mixing or masticating at some elevated temperature, which is usually greater than the melt temperature of the non-vulcanizing polymer component.
A TPV can be characterized by finely divided rubber particles dispersed within a plastic matrix. These rubber particles are typically crosslinked to promote elasticity. The physical properties of the rubber phase and those of the plastic matrix can influence the physical properties of the resulting TPV, in addition to other factors, such as the size and the concentration of the particles of the discontinuous phase, the presence of fillers, curatives, and other additives present in the composition, and the like. However, TPV compositions generally exhibit the processing characteristics of the plastic and the elasticity of the rubber, and are capable of providing many of the desirable properties of thermoset elastomers, yet are processable as thermoplastics.
Ethylene-based elastomers such as ethylene-propylene (alpha-olefin)-diene (EPDM) are generally polymers of very high molecular weight (as measured by their Mooney viscosity), and are often suitable for use in TPV applications. Many TPV compositions that utilize EPDM have varying amounts of a diene, which is usually one of 5-ethylidene-2-norbornene, 1,4-hexadiene, 1,6-octadiene, 5-methyl-1,4 hexadiene, and 3,7-dimethyl-1,6-octadiene. Such dienes have been found to provide peroxide cured TPV compositions with fair resistance to compression set among other mechanical properties. In TPV compositions having an EPDM polymer that is cured less than about 90 percent, compression set is generally unacceptably high for many applications, especially at elevated temperatures.
EPDM's containing vinyl norbornene (VNB), which is a non-conjugated diene having two polymerizable double bonds, are known from EP843698; EP843702 and EP843701. These polymers have long chain branching (LCB). High levels of LCB improve processability, but can impair physical properties of the final product after conversion of the polymer by extrusion or molding etc. The two double bonds are both capable of polymerization with olefins in the presence of transition metal catalysts.
The prior art describes the benefit of VNB over ethylidene norbornene (ENB). The VNB derived EPDM provides improved cure rate, cure state, and performance in free-radical curing, improved processability from the highly branched structure, and requires a low level of diene to provide suitable physical properties in the final product comparable to ENB derived EPDM.
WO99/00434 describes combining ENB, VNB and specific branching inhibitors to produce EPDM with reduced branching. The ENB derived units are present in amounts well in excess of the amount of VNB. The spectrum of LCB and molecular weight distribution (MWD) variations that can be obtained appear to be limited by the process characteristics (a branching modifier is used). Very low levels of branching can be hard to obtain because of cationic branching generated by the ENB. Broad MWD is favored.
It is known to make EPDM type polyolefins, generally those having ENB derived units, in a continuous stirred tank series reactor layout, primarily to obtain broader MWDs and the attendant processability benefits resulting therefrom. Reference is made to U.S. Pat. No. 4,306,041; EP227206; WO99/45047; and WO99/45062.
U.S. Pat. No. 6,319,998 and WO 99/45062 describe processes using metallocene type catalysts that have high activity and extremely efficient incorporation of diene. This leads to high levels of LCB, and in some cases the formation of gel.
Other references of interest can include: WO 99/00434, U.S. Pat. No. 6,207,756, WO 98/02471, U.S. Pat. Nos. 3,674,754, 4,510,303, 3,629,212, 4,016,342, 5,674,613, EP 1088855, U.S. Pat. No. 6,281,316, EP 784062, U.S. Pat. Nos. 4,510,303, 5,698,651 and 6,225,426.
There is still a need for yet further improvements in cure efficiency but without the debits incurred from too high a level of branching in VNB polymers.